The present invention relates to a monioring circuit for supervising different operational conditions of an electric motor and in particular it relates to a monitoring circuit of the type which employs a measuring resistance connected in series with the stator winding of the electric motor to produce a voltage drop which is proportional to the electric currrent flowing through the motor.
Monitoring circuits of this kind are known and find practical use for example in a vacuum cleaner so as to monitor load of the driving electric motor and disconnect the same when an overload occurs. In this case, the electric motor is controlled via a thyristor and the measuring resistance is connected in the thyristor circuit to provide a voltage which is proportional to the motor current. This actual value voltage is applied via a timing RC member to a voltage dividing potentiometer which in turn is connected to the base of a transistor. This transistor is connected parallel to the thyristor circuit. The voltage dividing potentiometer adjusts the actual value voltage to such a value that during normal operational conditions of the motor the transistor is not yet switched on. Only when the motor current increases due to an overload, then the actual value voltage across the measuring resistor increases accordingly and so does the voltage on the base of the transistor. As a consequence, the transistor switches on, blocks the thyristor and the electric motor is stopped. At the same time, another transistor is switched on to apply via a feedback conduit a signal to the base of the first-mentioned transistor so as to keep it in the switched on state. Hence, the entire control circuit is brought in a self-restraining condition. The driving motor can be restarted only by disconnecting and reconnecting supply voltage from the main. During the start of the electric motor when as known a momentary high current flows through the stator winding and consequently a high voltage drop appears across the measuring resistor, the aforementioned RC member prevents the first transistor from immediately disconnecting the electric motor. In this prior art arrangement the setting of the nominal value by the voltage dividing potentiometer is extremely critical inasmuch as the RC member simultaneously generates during the start of the motor a certain voltage across the voltage dividing potentiometer. Accordingly, the desired threshold value cannot be adjusted to any arbitrary magnitude because in the case of low threshold values reached during the starting operation are in the order of the voltage across the RC member, the transistor is switched on and restarts the motor. Therefore, the magnitude of the adjusted threshold value must be sufficiently large to be above the voltage value across the RC member. This limitation in prior art monitoring circuits is considerable disadvantageous.